how about changing it so you have a margin where the victor wounds the loser, but is unwounded themselves.
I've been considering making kill/wound checks separate, yes. That way you could end up with neither wounded/both wounded/one wounded/one wounded+one dead/both dead scenarios (though in the first case, it would just cycle the battle until one of the latter conditions happened).
The cheapest way for me to do this (in terms of not changing any stats) would just be to have them do the existing opposition roll twice, once to see if the attacker gets a hit, once to see if the defender gets a hit. They retain their relative bonuses based on whether they're attacking or defending and it's worked out based on that. These rolls would then be an Inflict roll vs a Deflect roll, modified by attack/defence position. If your Inflict roll beats their Deflect roll, you score a wound. If it exceeds the wound margin (which may go back up again as a result of balance), it's a kill.
A more painful way (in that I might have to consider adding specialist bonuses to more units) but ultimately perhaps better way would be to do this
and give units bonuses to surviving and inflicting wounds,
separate from their position on the offence/defence. If I do that, I'll probably change Weaponsmithy and Armoury to favour Inflicting/Deflecting instead of Attack/Defense.
What do we think?